Semiconductor nanocrystals (also known as Quantum Dot.TM. particles) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of semiconductor nanocrystals shift to the blue (higher energies) as the size of the nanocrystals gets smaller.
Bawendi and co-workers have described a method of preparing monodisperse semiconductor nanocrystals by pyrolysis of organometallic reagents injected into a hot coordinating solvent (Murray et al. (1993) J. Am. Chem. Soc., 115:8706). This permits temporally discrete nucleation and results in the controlled growth of macroscopic quantities of nanocrystals. Size-selective precipitation of the crystallites from the growth solution can provide crystallites with even narrower size distributions. The narrow size distribution of the semiconductor nanocrystals allows the possibility of light emission with narrow spectral linewidths.
In an effort to improve the photoluminescent yield of the semiconductor nanocrystals, the nanocrystal surface has been passivated by reaction of the surface atoms of the nanocrystal with organic passivating ligands, to eliminate energy levels at the surface of the crystallite that lie within the energetically forbidden gap of the bulk interior. These surface energy states act as traps for electrons and holes that degrade the luminescence properties of the material. Such passivation produces an atomically abrupt increase in the chemical potential at the interface of the semiconductor and passivating layer (see, Alivisatos (1996) J. Phys. Chem. 100:13226). Murray et al. (1993), supra, describes CdSe nanocrystals capped with organic moieties such as tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO) with quantum yields as high as 20% in organic solvents such as toluene (see, also, doctoral thesis of Christopher Murray, "Synthesis and Characterization of II-VI Quantum Dots and Their Assembly into 3-D Quantum Dot Superlattices" (1995) Massachusetts Institute of Technology; and Kuno et al. (997) J. Phys. Chem. 106(23):9869).
Although semiconductor nanocrystals prepared as described by Bawendi and co-workers exhibit near monodispersity, and hence, high color selectivity, the luminescence properties of the material is process dependent. The stability of the photoluminescent property of the nanocrystal is a function of the nature of the passivating species coating the outer surface of the nanocrystal. Known organically coated nanocrystals are not robust and exhibit degradation of photoluminescent yield in solution. This is likely due to dissociation of the passivating layer from the surface of the nanocrystal or degradation of the passivating layer resulting in degradation of the semiconductor surface.
Passivation of semiconductor nanocrystals using inorganic materials also has been reported. Particles passivated with an inorganic coating are more robust than organically passivated particles and have greater tolerance to processing conditions necessary for their incorporation into devices. Previously reported inorganically passivated semiconductor nanocrystal particle structures include CdS-capped CdSe and CdSe-capped CdS (Than et al. (1996) J. Phys. Chem. 100:8927); ZnS grown on CdS (Youn et al. (1988) J. Phys. Chem. 92:6320); ZnS on CdSe and the inverse structure (Kortan et al. (1990) J. Am. Chem. Soc. 112:1327); ZnS-capped CdSe nanocrystals (Hines et al. (1996) J. Phys. Chem. 100:468; ZnSe-capped CdSe nanocrystals (Danek et al. (1996) Chem. Malerials 8:173); and SiO.sub.2 on Si (Wilson et al. (1993) Science 262:1242).
Kortan et al. (1990), supra, describes a ZnS capped-CdSe nanoparticle that has a layer of thiolphenyl groups bound to the outer surface. The thiolphenyl groups were used to passivate the surface and to allow the clusters to be isolated in powder form. Lawless et al. (1995) J. Phys. Chem. 99:10329 reported the preparation of CdS semiconductor nanocrystals capped with bifunctional mercaptocarboxylic acids HS(CH.sub.2).sub.n COOH, wherein n is 1-3. TiO.sub.2 particles were attached to the CdS nanocrystals through the functional carboxylic acid group of the bifunctional capping moiety in order to promote interparticle electron transfer between dissimilar semiconductor particles.
The semiconductor nanocrystals described above are soluble or dispersible only in organic solvents, such as hexane or pyridine. Many applications that rely on the fluorescent emission of the semiconductor nanocrystals require that the semiconductor nanocrystals be water-soluble.
Many reported water-soluble semiconductor nanocrystals suffer from significant disadvantages that limit their wide applicability. For example, Spanhel et al. (1987) J. Am. Chem. Soc. 109:5649, discloses a Cd(OH).sub.2 -capped CdS sol; however, the photoluminescent properties of the sol were pH dependent. The sol could be prepared only in a very narrow pH range (pH 8-10) and exhibited a narrow fluorescence band only at a pH of greater than 10. Such pH dependency greatly limits the usefulness of the material; in particular, it is not appropriate for use in biological systems.
Other groups have replaced the organic passivating layer of the semiconductor nanocrystal with water-soluble moieties; however, the resultant derivatized semiconductor nanocrystals are not highly luminescent. Short chain thiols such as 2-mercaptoethanol and 1-thio-glycerol have been used as stabilizers in the preparation of water-soluble CdTe nanocrystals. See, Rogach et al. (1996) Ber. Bunsenges. Phys. Chem. 100:1772 and Rajh et al. (1993) J. Phys. Chem. 97:11999. Other more exotic capping compounds have been reported with similar results. See, Coffer et al. (1992) Nanotechnology 3:69 which describes the use of deoxyribonucleic acid (DNA) as a capping compound. In all of these systems, the coated semiconductor nanocrystals were not stable and photoluminescent properties degraded with time.
The unavailability of aqueous suspensions or solutions of semiconductor nanocrystals with sharp photoluminescent emissions limits their application in a variety of water-based applications, such as biological applications. In addition, aqueous solutions can often be very aggressive chemical systems and many of the known water-soluble semiconductor nanocrystal systems degrade, mainly by photoanodic decomposition at the semiconductor surface interface, during long exposure times in water.
Thus, there remains a need for water-soluble semiconductor nanocrystals that can be prepared as stable, robust suspensions or solutions in aqueous media. There is also a need for water-soluble semiconductor nanocrystals capable of energy emission with high quantum efficiencies, which possess a narrow particle size (and hence with narrow photoluminescence spectral range).